This does the following :

- opening a local image file, using
`pixpipe.FileImageReader`

- converting it into a
`pixpipe.Image2D`

object - compute the dx and dy derivative with
`pixpipe.ImageDerivativeFilter`

- feed a
`pixpipe.GradientImageFilter`

with the derivative - displaying gradient direction and magnitude with a
`pixpipe.CanvasImageWriter`

Select an image file:

The first image is the gradient direction. Note that it does not really make sens to match a [-PI/2, PI/2] interval to [0, 255].

The second image is the gradient magnitude, matched to [0, 255]